1. Field of the Invention
The present invention relates to open low-density nonwoven abrasive articles, and methods of making same, including a fibrous web treated to have resistance to phenol attack and formed of polyamide staple fibers having low tenacity.
2. Description of the Related Art
Nonwoven abrasive articles are known and have been described, for example, in U.S. Pat. No. 2,958,593 (Hoover et al.), and generally comprise fibers formed into a nonwoven web provided as a foraminous three-dimensional integrated network structure with fine abrasive particles and curable binder attached thereto. Such nonwoven abrasive articles are useful in discrete sheet form as well as in various converted forms, such as wheels, discs, and flap brushes. In these converted forms, the resulting articles are useful to scour, clean, condition, and/or decorate the surfaces of such materials as metal, wood, plastics, glass, ceramics, and composites.
The fibers that have been used in the known nonwoven abrasive articles are formed from various polymers, including polyamides, polyesters, polypropylene, polyethylene, and various copolymers. Naturally occurring fibers such as cotton, wool, bast fibers, and various animal hairs may also be suitable. Suitable abrasive particles can be formed of flint, garnet, aluminum oxide, diamond, silicon carbide, etc. Binders commonly comprise cured versions of hide glue or varnish, or one or more resins such as phenolic, urea-formaldehyde, melamine-formaldehyde, urethane, epoxy, and acrylic resins. Phenolic resins include those of the phenol-aldehyde type. Prior abrasive nonwoven constructions include SCOTCH-BRITE.TM. products sold by 3M Company, St. Paul, Minn., of a type requiring the use of solvent-coated cross-linked urethanes in the prebond to provide the requisite elasticity and protect nylon fibers of the web from attack by subsequently applied phenolic make coates used for bonding of mineral abrasive into the web.
Nonwoven abrasive articles have been made by the following generally known scheme. A "prebond" coating of a binder precursor solution without containing abrasive particles, which includes one or more of the above-named resins, is coated on the web and cured by exposure to heat in order to impart sufficient strength to the nonwoven web for further processing. Then, a "make" coating based on a resinous organic binder is applied to the web to secure fine abrasive grains throughout the lofty fibrous mat and cured. Thereafter, a "size" coating of resinous binder material and abrasive particles is applied, usually by spray-coating, over the prebonded web to increase the abrasive characteristics of the article, such as preventing the abrasive mineral from shelling. Then, the size coating is cured. The resins of the various prebond, make, and size coatings could be the same or different, depending on the various web and abrasive particle properties desired.
Phenolic resin binders, in particular, are used extensively to manufacture nonwoven abrasive articles because of their thermal properties, availability, low cost, and ease of handling. The monomers currently used in greatest volume to produce phenolic resins are phenol and formaldehyde. Other important phenolic starting materials are the alkyl-substituted phenols, including cresols, xylenols, p-tert-butylphenol, p-phenylphenol, and nonylphenol. Diphenols, e.g., resorcinol (1,3-benzenediol) and bisphenol-A (bis-A or 2,2-bis(4-hydroxyphenyl)propane), are employed in smaller quantities for applications requiring special properties.
There are two basic types of phenolic resins: resole and novolak phenolic resins. Molecular weight advancement and curing of resole phenolic resins are catalyzed by alkaline catalysts. The molar ratio of aldehyde to phenolic is greater than or equal to 1.0, typically between 1.0 and 3.0. In the production of adhesive coatings for nonwoven abrasives, one standard starting phenolic resin composition is a 70% solids condensate of a 1.96:1.0 formaldehyde:phenol mixture with 2% potassium hydroxide catalyst added, based on the weight of phenol. The phenolic resin composition is typically 25-27% by weight water and 3-5% by weight propylene glycol methyl ether, which were thought required to reduce the viscosity of the resin of the conventional phenolic formulations. Before this resin is used as a component of a make or size coating, i.e., to make it coatable, further viscosity reduction is often achieved by addition of volatile organic compounds, which are commonly referred to by the abbreviation "VOCs". A conventional binder precursor solution containing a phenolic resin which is used to produce a prebond coating for a nonwoven web contained up to 40% by weight of a VOC, such as isopropyl alcohol, to reduce the viscosity and make the phenolic resin compatible with other binder components, while a binder precursor solution which was used to produce a size coating might contain up to 20% by weight of a VOC, such as diethylene glycol ethyl ether.
In order to reduce emissions of VOCs, it has been suggested to increase the water compatibility of phenolic resins. J. D. Fisher, in an article entitled "Water Compatible Phenolic Resins" in Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering; No. 65, pp. 275-276 (1991), describes methods of making "water compatible" phenolic resins, their benefits, and their shortcomings. However, it would be desirable to be able to adjust the water compatibility of the binder without the need for fastidious management and oversight of the cure system dynamics or the need for additional operations and equipment to chemically synthesize a functionalized or otherwise structurally altered phenolic binder molecular structure.
Also, a compatibility problem arises from the use of the phenolic binder in particular together with a nonwoven web based on polyamide fibers. A particularly useful known nonwoven abrasive article is one comprising a web of polyamide fibers and resole-type phenolic resins as the curable binder. Such a composition provides for strong, tough, temperature resistant abrasive articles that may be made economically. However, as a drawback, it is known that free phenol, which is typically present in resole phenolic resins, can chemically attack and thereby weaken such polyamide fibers. One adverse effect of this corrosive phenol attack on the poly, amide fibers is the embrittlement of the fibers, which entailed a loss of flexibility, resilience, elongation and the like in the fibers individually and the web as a whole. Prior to the present invention, the practice employed to alleviate this problem was to use polyamide fibers that have been highly drawn. Such fibers exhibit a tenacity typically in the 5.5 g/denier to more than 8 g/denier range. The morphology of such highly-drawn fibers tends to be more crystalline, and both amorphous and crystalline regions are highly oriented. Such morphology provides a barrier to the rapid attack of free phenol since the diffusion rate of phenol into the fiber is substantially decreased. While this method overcomes the problem of phenol attack, other problems are presented in processing. For example, it is more difficult to impart a stable crimp in such highly drawn fibers and, therefore, the processing into a nonwoven web can be more difficult. In addition, the additional processing equipment and supervision required to manufacture such highly oriented fibers is significant from an economic standpoint. It would be highly attractive to the industry to be able to employ intermediate and low tenacity low tensile strength polyamide fibers which need not be subjected to special orientation enhancing procedures, such as having a tenacity below 5 g/denier, while otherwise preventing the phenol attack on the fibers.
Further, as with other conventional phenolic systems for binding fibrous webs, the resole phenolic system employed to consolidate polyamide fiber nonwovens, generally requires the use of at least some volume of VOCs. Such VOCs are typically removed during the manufacturing process and must be recovered or otherwise treated to avoid or minimize atmospheric release. It would be desirable to reduce, if not prevent, the use of VOC solvents in the polyamide fiber/resole phenolic system due to the added costs and inconvenience associated with handling and disposing the VOC solvents. Further, the VOC solvents are thought to aggravate or assist phenol attack of the surfaces of the polyamide fibers.
Modified phenolic resins that provide for reduced VOC emissions for use in nonwoven abrasive articles are likewise known and are described in, for example, commonly assigned U.S. Pat. No. 5,178,646 (Barber, Jr. et al.), where poly(oxyalkylene)amine and urea compounds are employed for this purpose. Rubber-modified phenolic resins have also been used in the manufacture of nonwoven abrasive articles, such as in the disclosure of commonly assigned U.S. Pat. No. 2,958,593 (Hoover et al.), as an optional rubber treatment disposed on one side of the structure to increase the resistance of the overall abrasive article structure to tearing and shredding. For example, Hoover et al. exemplifies a nylon fiber web being first coated with a phenol-formaldehyde and amine terminated polyamide resin-containing coating, followed by transmitting the phenol exposed web to a curing oven where the coated web is so heat-treated such that the emitted treated web is cured to a nontacky state while still warm, and, only thereafter, a rubbery composition based on a butadiene acrylonitrile copolymer latex (viz. trade designation "Hycar.RTM.Latex 1561", from B. F. Goodrich Co.) is applied to the opposite side of the web and heat-cured in an oven. In the disclosed arrangement of Hoover et al., the nylon fibers would be exposed and contacted with phenol without any prior fiber orientation or modifier ingredients present at that time being identified therein to counteract phenol attack on the nylon fiber surfaces. Similarly, commonly assigned U.S. Pat. No. 4,189,395 (Bland) discloses a cleansing pad, which slowly releases its surfactant loading in use over an extended period of time. This cleansing pad of Bland comprises a pad impregnated, in a final treatment during manufacture, with a composition comprising a water-insoluble cured acrylic resin having a grease-cutting, suds-forming nonionic surfactant blended therein. The acrylic resin disclosed by Bland can be a latex commercially available under the trade designation "Rhoplex" by Rohm and Haas Co. and "Hycar.RTM.H2671 " available from B. F. Goodrich Co. However, the web employed by Bland is disclosed as first being integrated with a binder such as a thermosetting resin comprising phenol-formaldehyde before the web is subjected to the separate later treatment with the surfactant-containing composition. Also, commonly assigned U.S. Pat. No. 4,018,575 (Davis et al.) discloses a low-density abrasive article comprising a fibrous web composed of glass filament bundles. The glass filament bundle web of Davis et al. is disclosed as being prebonded with preferred prebonding resins of polyacrylates, butadiene-acrylonitrile rubbers such as are sold under the trade designation "Hycar.RTM.1562", and polyurethanes. Glass fibers are not subject to phenol attack.
The modification of a phenolic resin precursor system used for binding nonhighly drawn, lower tenacity polyamide web fibers by the presence of a modifier agent therewith which alleviates, if not prevents, the degradation of polyamide fibers in the presence of phenol is not thought to have been known prior to the present invention.